Akt (or protein kinase B (PKB)) is a well-characterized serine/threonine kinase that promotes cellular survival. Akt is activated in response to many different growth factors, including IGF-I, EGF, bFGF, insulin, interleukin-3, interleukin-6, heregulin, and VEGF (1). Akt is the cellular homologue of the product of the v-akt oncogene (2-4) and has 3 isoforms, Akt 1, 2, and 3 (or PKB α, β, and {tilde over (γ)}). Activation of all three isoforms is similar in that phosphorylation of two sites, one in the activation domain and one in the C-terminal hydrophobic motif, are necessary for full activity.
For Akt 1, phosphorylation of T308 in the activation domain by PDK1 is dependent on the products of phosphatidylinositol (PI) 3-kinase (PI3-K), phosphatidylinositol 3,4 bisphosphate (PIP2) and phosphatidylinositol 3,4,5 trisphosphate (PIP3). Cellular levels of PIP2 and PIP3 are controlled by the tumor suppressor, dual-phosphatase PTEN, which dephosphorylates PIP2 and PIP3 at the 3′-position. The mechanism of S473 phosphorylation is less clear. Kinases potentially responsible for S473 phosphorylation include PDK1 (5), ILK or an ILK-associated kinase (6, 7), Akt itself (8) or an as yet uncharacterized PDK2. Akt activation may also be achieved through PI3-K independent means, by phosphorylation of Akt by kinases such as PKA (9) or CAM-KK (10). Once activated, Akt exerts anti-apoptotic effects through phosphorylation of substrates that directly regulate the apoptotic machinery such as Bad (11, 12) or caspase 9 (13), or phosphorylation of substrates that indirectly inhibit apoptosis such the human telomerase reverse transcriptase subunit (hTERT) (14), forkhead transcription family members (15, 16), or IκB kinases (17, 18).
Functionally, Akt promotes survival in vitro when cells are exposed to different apoptotic stimuli such as GF withdrawal, UV irradiation, matrix detachment, cell cycle discordance, DNA damage, and administration of anti-Fas antibody, TGF-β, glutamate, or bile acids (19-33). In vivo, activation of the PI3K/Akt pathway contributes to tumorigenesis in many types of tissues, including breast, ovarian, brain, prostate, and lymph tissues (34). It has been shown that Akt is constitutively active in over 90% of NSCLC cell lines and contributes to both chemotherapeutic resistance and radiation resistance (35). In addition, it has been shown that Akt is constitutively active in many breast cancer cell lines, and serves a similar function in promotion of cellular survival and chemotherapeutic resistance (36).
The foregoing shows that inhibitors of Akt would be desirable for preventing or treating a number of diseases, especially diseases such as cancer.
The advantages of the invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.